Shader artifacts rendering in some spots of the mesh
Those artifacts are showing up for a number of reasons.  The first is that you have polygons that are very non-planar and this will usually cause artifacts for lighting.

Here is an example that shows a quad but it's folded sharply and two distinct triangles can be seen when viewed along the plane of the quad.  Try and make sure to keep quads as flat as possible.  Non-planer quads will also give you problems with texture mapping so it's something to always look for and repair.



Another problem you are having is that you are rendering with a very low polygon model.  Cycles has difficulty dealing with models which are not subdivided at least a couple times.  It's best to use a subdivision surface (subsurf) modifier to handle this since it can be removed without affecting the base model or it can be applied so the model will be permanently sub-divided.
Here is a sequence of images that show the effect that various levels of subdivision can have on rendering artifacts.



Another way to deal with these rendering issues is to use more lights.  Global Illumination will also help since these areas will receive indirect light that will also help smooth things out.
The following image shows the effect of changing the angle of a single light source in order to show how the area is affected by direct illumination.


For the final image here, I did not alter the mesh at all but instead only relied on the Sub-Division Modifier at 1 iteration and also there are 3 lights as well as an environment map.  The ground also adds a bit of indirect light that helps to fill in the lighting for areas that the other lighting is not reaching. 
This all adds up to a mesh that looks pretty decent even though the bash mesh is low poly and it has some fairly substantial non-planer errors.  If the Sub-D modifier is increased to use 2 iterations; the few problem areas that are still showing should no longer be perceptible.